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How to calculate the electricity consumption to buy a generator for a house?

Electric generators or generator sets are very useful devices , which come in different models and with different capacities. That is why when selecting the most suitable for a house , several criteria must be taken into account. The main criterion is the electricity consumption generated in the home, which must be covered by the generator to be purchased.

How to calculate the electricity consumption to buy a generator for a house?

In this article we will describe in detail each of the aspects to consider when buying an electric generator , in order to  calculate the electrical consumption  of the appliances, tools and other equipment that work in a home.

Physical units and their conversions

We must know very well the physical units related to the electrical consumption of the devices, in order to carry out the calculations correctly.
Let’s first review the terms of:

  • Amperage (A) : is the electrical current that a device requires to operate.
  • Voltage (V) : is the electrical voltage supplied by power plants to companies and houses. This can vary by country, and ranges from 100V to 440V.
  • Watt (W) : is the unit of electrical power.

To calculate electrical power (W), that is, the electrical consumption of a device, the electrical current (A) is multiplied by the voltage (V): A * V = W
Generally, the power of devices is expressed in watts, but when power is high, multiples such as kilowatt (kW) or megawatt (MW) are often used.
1kW = 1,000W
1MW = 1,000,000W

  • Kilovolt-ampere (kVA) : is a multiple of the volt-ampere (VA), which is the unit of apparent power and complex power of an electrical device.

The kVA is the unit used to indicate the power of an electric generator .

How to calculate the electrical power of a device?

To calculate the electricity consumption of our entire house, we must start by knowing how the electricity consumption of a single device or electrical appliance is calculated.
Taking into account the definitions and relationship of the physical units already mentioned, we will go on to present an example:

Suppose we have a laptop with 1A at 220V, applying the formula to calculate the electrical power, we will obtain the consumption of this device: 1A x 220V= 220W

Usually, electrical equipment has a label or sheet where its electrical power is indicated, so we would not have to calculate it. However, there are also devices that only indicate their amperage (A) and voltage (V), in which case we must apply the indicated formula to know the electrical power of said devices.

In the event that one or more of the devices that we have in our home has a high electrical power, we will find that it is expressed in kW or MG. In either case, we would only have to convert these values ​​to W, using the equivalence values.
For example, suppose we have a powerful tool whose label indicates that its power is 350kW. To convert this value to watts, we just have to multiply it by the equivalence: 350kW * 1000 = 350,000W

The following table shows some devices, with their respective electrical power:

Apparatus, equipment or tool Electrical power electrical consumption (W)
Fluorescent lamp 35
100 watt bulb 100
VCR 100
Refrigerator / Fridge * 800
Freezer / freezer * 700
Microwave oven 1500
Coffee maker 1200
Hair dryer * 1500
Radio 200
Television 250
Computer 400
Printing machine 350
Bench grinder * 400
Hedge trimmers * 500
Chainsaw, 25 to 40 cm blade length * 1500
Hand lamp 500
30 cm diameter emery 2500
Drill * 700
12.7 cm diameter circular saw * 1000
hand sander * 1020
20 cm diameter floor sander * 2000
Pressure Washer / Hydrojet, 1 Hp 3500
20 Hp compressor * 2200

Equipment with inductive power

Some electrical devices have rotary motors for the moment of starting, and at that moment they require inductive power, which is much higher than the electrical power they require to work, once they have started. An example of this equipment is washing machines, refrigerators, pumps , among others (see devices with * in the table above). For some of these devices, the required inductive power is 3 or 6 times higher than the standard operating power. This is why the calculation of electrical consumption for these equipments must be different.

Depending on the type of startup of the equipment, its consumption must be calculated:

  • Direct Start (DOL) : the standard electrical power is multiplied by 6.
  • Star-Delta (ET) starting : the standard electrical power is multiplied by 3.
  • Soft Starter or Variable Frequency (VF) Starter : the standard electrical power is multiplied by 2.5.

Let’s take the following examples as a guide:
1 Fridge = 375W x 3 = 1125W
1 Water Pump (1HP) = 735w x 3 = 2205W

To facilitate this calculation, we will show in the following table the values ​​corresponding to the inductive power, according to the horsepower (HP) of some devices.

Engine Power (HP) Consumption in continuous use (W) Starting power (W)
1/8 275 850
1/6 275 850
1/4 400 1050
1/3 450 1350
1/2 600 1800
3/4 850 2600
1 1100 3300
two 2200 660

How to calculate the electricity consumption in our house?

Once we know the ways to calculate the electrical consumption of the devices, we can proceed to calculate the electrical consumption in our home. For this we only need to make a list with each and every one of the devices, equipment or appliances that work with electricity, and take note of the electrical power of each of them.
Once the list is obtained, we must verify that all the electrical power values ​​are expressed in the same unit (watts W, for example). Otherwise, we must make the relevant conversions, in order to continue.
Having all the values ​​in the same power unit, we must add them to obtain the total value. In the following table, we will show an example:

Quantity Apparatus, equipment or tool Electrical power – electrical consumption (W) inductive power factor Overall (W)
10 100 watt bulb 100  NA  1000
1 Refrigerator / Fridge 800 3  2400
1 Microwave oven 1500  NA  1500
1 Coffee maker 1200  NA  1200
1 Hair dryer 1500 3  4500
two Television 250  NA  500
1 Computer 400  NA  400
1 30 cm diameter emery 2500  NA  2500
1 Drill 700 3  2100
1 planer 700 3  2100
Total electrical consumption of the house  18200

The sum of the power required by the appliances is 18,200 W.
This value is converted to kW by dividing it by 1,000: 18,200/1,000 = 18.2 kW.

Then, to obtain the required KVA, the value expressed in kW is divided by the cosine of Fi: 18.2 / 0.8 (Cosine Fi(F)) = 22.75 KVA

When obtaining the final value, it is recommended to increase it by 20% to have a safety margin: 22.75 + 20% = 27.3 KVA

In conclusion, an electric generator that covers 27.3 KVA is needed.

Advice

  • It is recommended not to start all the devices at the same time, once they are connected to the electric generator .
  • Try to keep connected to the generator , only those devices that are in use.
  • Perform proper maintenance on your electric generator .